1. Field of the Invention
The present invention concerns a method for acquiring dynamic magnetic resonance signals in magnetic resonance tomography.
2. Description of the Prior Art
Adaptation of the temporal resolution of the measurement of speeds of signal variations is necessary in the acquisition of dynamically varying magnetic resonance signals that are used to generate a tomographic image. Faster signal variations thus must be acquired with higher temporal resolution than slower varying signals. A fast signal change occurs, in particular shortly after the injection of a contrast agent into the subject, while the signals vary substantially slower after what is known as the first-pass effect, i.e., as soon as the contrast agent has, for example, passed the liver of a patient. It is desirable to adapt the temporal resolution to the speed of the signal variation during the signal acquisition. Moreover, as is always the case, unwanted signal variations (as are created, for example, by a movement or respiration of the patient) generate measurement artifacts and therefore must be compensated or prevented.
One possibility for reducing measurement artifacts due to movements or breathing by the patient is to implement the entire acquisition (scan) with the highest possible temporal resolution. Movements of an organ that are created due to breathing or movement of the patient then are not of importance during the acquisition of signals (data) for a single image since they ensue distinctly slower than the image acquisition. The influence of movement on a series of images is visible, however, such that in that case correction measures are necessary. In particular, in the case of movements perpendicular to the slice plane these correction measures are possible only with three-dimensional, with the temporal resolution being substantially clearly less for purely two-dimensional measurements. Additionally a large data quantity results due to the high temporal resolution, particularly for longer examinations.
Another possibility for preventing measurement artifacts due to breathing of the patient is to implement the measurement during a breath-hold by the patient. The signal is acquired with the highest possible temporal resolution during the breath-hold. However, after a relatively short scan duration a pause is necessary so that the patient can breath again. It is nearly impossible to repeat the scan with the patient in the identical respiratory position as before, during a subsequent breath-hold by the patient. Elaborate correction methods are in turn necessary for compensation of the different respiratory positions respectively in successive scans.
Furthermore, it is possible to acquire the breathing motion (for example using a navigator echo) in a discrete measurement and to use it for correction of the actual measurement data. This method requires a high time expenditure because the breathing motion must be acquired in a separate measurement. Moreover, it is difficult to correct the complex three-dimensional movement of the organs during a breathing cycle.
Furthermore, it is possible to combine various methods. Thus the quickly varying signals can be acquired first with the highest possible temporal resolution during a breath-hold by the patient, and after this the slowly varying signals are acquired during free respiration of the patient. In principle the same difficulties explained above still occur particularly during the signal acquisition during free respiration.